The invention relates generally to exhaust sensors. Particularly, the present invention relates to a method of treating an electrolyte and of forming an exhaust sensor.
The automotive industry has used exhaust sensors in automotive vehicles for many years to sense the composition of exhaust gases, e.g., oxygen, hydrocarbons, and nitrous oxides. For example, sensors are used to determine the exhaust gas content for alteration and optimization of the air to fuel ratio for combustion.
A sensor typically has a conductive solid electrolyte disposed between porous electrodes. In the use of a sensor for automotive exhaust, the unknown gas is exhaust and the known gas (i.e., reference gas) is usually atmospheric air because oxygen content in the air is relatively constant and readily accessible. This type of sensor is based on an electrochemical galvanic cell operating in a potentiometric mode to detect the relative amounts of oxygen present in an automobile engine""s exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force (emf) is developed between the electrodes according to the Nerst equation:       E    =                  (                  RT                      4            ⁢            F                          )            ⁢              xe2x80x83            ⁢              ln        ⁡                  (                                    P                              O                2                            ref                                      P                              O                2                                              )                                                  where:                                              E          =                      electromotive force                                                        R          =                      universal gas constant                                                        F          =                      Faraday constant                                                        T          =                      absolute temperature of the gas                                                                    P                          O              2                        ref                    =                      oxygen partial pressure of the reference gas                                                                    P                          O              2                                =                      oxygen partial pressure of the exhaust gas                              
A sensor comprises an electrolyte disposed between a sensing electrode and a reference electrode. The electrolyte typically comprises any material conventionally employed for sensor electrolytes, including but not limited to, zirconia, such as yttria doped zirconia, ceria, strontium cerium oxide, barium cerium oxide, strontium cerium zirconates, and the like, and mixtures thereof. The electrodes on the other hand, typically comprise metals such as platinum, gold, palladium, rhodium, iridium, osmium, ruthenium, and mixtures and alloys comprising at least one of the foregoing, and other metals. Combined with the metals are metal oxides such as ceria, calcia, yttria, magnesia, lanthana, and mixtures and alloys comprising at least one of the foregoing.
A sensor is typically formed by disposing an electrolyte between and in intimate contact with a pair of electrodes such as a sensing electrode and a reference electrode to form an assembly. This assembly is then fired to a certain temperature as part of the forming process. During firing, certain impurities such as silica, sodium, and the like, accumulate on the surface of the electrolyte and the electrodes. These impurities affect the overall performance of a sensor. In particular, the surface composition of the electrolyte and the electrode are affected causing poor sensor performance.
With conventional sensor formulations, the desired surface composition is often not attained due to impurities introduced during the firing process at very high temperatures. Therefore, to remove the impurities, several methods have been used to treat the surface of the electrolyte and the electrode. Some of the treating methods include ion milling, electrical aging, hydrogen fluoride (HF) etching, and the like.
For example, ion milling a conical sensor is not very effective because only the outer surface of the sensor can be reached leaving the interior of the sensor untreated. The untreated area may retain impurities causing a low surface composition. Similarly, ion milling other sensors such as a flat plate sensor or a wide range sensor is not practical because of erosion of the porous protective layer on the outer surface.
With the electrical aging treatment method, the treatment advantageously changes the electrical properties of the electrode, however, impurities continue to exist on the electrodes, resulting in poor sensor performance. Essentially, electrical aging affects the boundary layer between the platinum electrode and the ionically conductive zirconia body. Poor xe2x80x9cconnectionsxe2x80x9d between the platinum electrodes and the yttria-zirconia electrolyte inhibit oxygen ions from efficiently entering into (inner electrode) and coming out of (outer electrode) the yttria-zirconia electrolyte. The rate of oxygen ion transport is important to the functioning of the sensor. High voltage minimum indicates not enough oxygen ions formed or transferred to the yttria-zirconia from the inner electrode, while low voltage maximum indicates the oxygen ions are not being used effectively at the outer electrode.
As the electrode is electrically aged the charge causes cationic impurities such as sodium to migrate towards the (xe2x88x92) pole. The anionic impurities such as chlorine are migrated towards the (+) pole. Alternating current drives both anions and cations towards both the surface and the boundary layer. The zirconium oxide at the boundary layer can break its bonds to platinum and be converted to non-ionically conductive materials such as zirconium oxychloride or sodium zirconium oxide. The yttrium can be converted to yttrium chloride the platinum surface can be converted to platinum chloride.
Hydrogen fluoride etching is capable of removing some surface impurities primarily metal oxides such as silica, alumina, yttria, and zirconia, capable of dissolving both metals and metal oxides. However, the rate of dissolution for metal oxides is orders of magnitude faster than for metals. For example, XPS of some poisoned samples detected the metals copper, silver, indium and lead present in the platinum sputtered electrode. A 2-wt % hydrogen fluoride treatment for 15 seconds was applied to 50 sensors. The hydrogen fluoride solution was evaporated and residue analyzed. The oxides of silica, alumina, yttria and zirconia were detected, while the metals copper, silver, indium and lead were not detected. The same sensors were hydrogen fluoride treated several times for different lengths of time. Eventually the platinum electrode completely de-bonded and the sensor was no longer functional. XPS analysis of the de-bonded platinum showed the metallic impurities still present. Consequently, either the impurities are not removed from all sensor areas or a portion of the sensor can be damaged due to excessive etching.
There exists a need in the art for treating an oxygen sensor that can reduce material and process impurities, and which will help provide a desired electrode surface composition and electrolyte surface composition to attain optimal sensor performance.
The above-discussed issues and deficiencies are overcome with the methods of forming a sensor. In one embodiment a sensor is formed by forming an electrolyte. The electrolyte is densified and exposed to an alkaline solution comprising potassium hydroxide and/or barium hydroxide. An assembly is formed by disposing the electrolyte between and in intimate contact with a first electrode and a second electrode.
In another embodiment, the sensor can be formed by disposing an electrolyte between an in intimate contact with a first electrode and a second electrode to form an assembly. The assembly is heated and exposing to an alkaline solution. In this embodiment, the first electrode comprises up to about 5 wt % rhodium and up to about 99.5 wt % platinum, based upon the weight of the electrode.